The present invention is directed toward air pollution control equipment, and more particularly toward an apparatus and method for recovery of volatile organic compounds (VOC) from an air stream.
Pollution of the atmosphere by VOC is a pervasive problem. Control of VOC emissions from large, discreet sources can be accomplished relatively efficiently and economically. Much more difficult to tackle has been the problem of VOC emissions from smaller discrete sources dispersed within a large facility. For example, an automobile assembly plant or other large scale manufacturing facility may engage in operations such as degreasing and painting in remote locations. In some cases, the plant operators simply vent the VOC contaminated air stream to the atmosphere without further treatment. Not only does this have a detrimental impact on the environment, it may also be a violation of state and federal air pollution laws. The prior art has tended toward pollution control apparatus associated with each remotely located source of VOC contaminated air. Some facilities attempt to capture the VOC for disposal. Others attempt to recover the VOC for reuse. Those facilities capturing the VOC for disposal are subject to more stringent emission control standards than those recovering the VOC for reuse.
Methods and apparatus for recovering VOC contaminants from a gas stream are known in the art. Representative is Grasso, U.S. Pat. No. 5,198,000. Grasso discloses subjecting a VOC laden air stream to an absorption apparatus such as a packed column using a scrubbing liquid which absorbs the VOC from the gas stream and discharges the treated gas into the atmosphere. Grasso further teaches that the VOC laden scrubbing liquid is subjected to a separation apparatus which may be a distillation column, stripping column, evaporator, membrane stripping device or the like. The separated scrubbing liquid is recycled to the absorption apparatus and the VOC is captured for reuse or disposal.
While the apparatus of Grasso has the advantage of capturing the VOC for reuse and thus qualifying for less stringent emissions standards associated with recovery processes, Grasso contemplates associating an entire system of absorption, separation and recovery with each source of a VOC contaminated gas stream. The problem with such an apparatus and method is that the separation apparatus typically operates most efficiently when it can be run continuously. Continuous operation may not be possible if the absorption apparatus requires servicing or if the VOC laden air stream is not continuously produced. In addition, economies of scale can be achieved by providing large volumes of VOC laden scrubbing liquid for separation. Providing a centralized apparatus as taught by Grasso to which VOC contaminated gas streams are transported via ducts from remote locations is not a viable solution because of the large volume of the ducts necessary for such transport, the problem of VOC deposition within the ducts and the possibility of leaks of harmful VOC from the ducts to within a facility.
The present invention is directed toward overcoming one or more of the problems discussed above.
A first aspect of the present invention is an apparatus for removing VOC from a plurality of remotely located sources of VOC contaminated gas streams. The apparatus includes a plurality of liquid absorbers, such as stripping towers, each located near and communicating with a source of a VOC contaminated gas stream. The liquid absorbers associate the VOC with a scrubbing liquid. A conduit is coinected to each liquid absorber for conveying VOC laden scrubbing liquid from the stripping tower to a separator in fluid communication with each of the conduits. The separator receives the VOC laden scrubbing liquid from the plurality of liquid absorbers and separates the VOC from the scrubbing liquid. The apparatus may further include a conduit for recycling the scrubbing liquid from the separator to at least one of the plurality of stripping towers. A scrubbing liquid reservoir may be provided in fluid communication with the separator for receiving the separated scrubbing liquid. A recirculating conduit may further be provided in fluid communication between the scrubbing liquid reservoir and an inlet to the separator with a valve in fluid communication with the recirculating conduit selectively recirculating separated scrubbing liquid to the separator. The separator is preferably a distillation column and the liquid absorber is preferably a stripping tower. A heat exchanger may be provided in operative association between the separated scrubbing liquid and the VOC laden scrubbing liquid to be received in the distillation column for transferring heat from the separated scrubbing liquid to the VOC laden scrubbing liquid before it enters the distillation column. A buffer reservoir may be provided in fluid communication between the VOC laden scrubbing liquid conduits and the separator. A regulator provides a select rate of flow of VOC laden scrubbing liquid from the buffer reservoir to the separator. In a highly preferred embodiment, the scrubbing liquid reservoir is in fluid communication with the buffer reservoir and a valve in fluid communication with the recirculating conduit selectively recirculates separated scrubbing liquid to the buffer reservoir as required to maintain a select rate of flow of scrubbing liquid and VOC laden scrubbing liquid to the separator.
A second aspect of the present invention is a method for removing VOC from a plurality of remotely located sources of gas streams contaminated with VOC. Tile method includes contacting each VOC contaminated gas stream with a scrubbing liquid close to the VOC containing gas stream source to associate the VOC with the scrubbing liquid. The VOC laden scrubbing liquid is conveyed to a single separator. There the VOC is separated from the scrubbing liquid. The separation of the VOC from the scrubbing liquid may be accomplished by distillation. If so, heat is preferably exchanged between the scrubbing liquid separated from the VOC and the VOC laden scrubbing liquid before distillation of the VOC laden scrubbing liquid. The separated scrubbing liquid is preferably recycled to repeat the contacting, conveying and separating steps. The separated scrubbing liquid may be collected in a first reservoir and selectively recirculated from the first reservoir by combining it with VOC laden scrubbing liquid before the separating step. The VOC laden scrubbing liquid may be stored in a second reservoir before the separating step. The release of the VOC laden scrubbing liquid is controlled from the second reservoir to the separating step to provide a substantially constant rate of delivery of VOC laden scrubbing liquid to the separating step. In a highly preferred embodiment, VOC laden scrubbing liquid is provided from the second reservoir to the separating step at a substantially constant select rate and scrubbing liquid from the first reservoir is provided to the second reservoir as required to maintain a select rate.
The apparatus and method for recovery of volatile organic compounds of the present invention allows for capturing and conveyance of VOC in a liquid state, thereby dramatically decreasing the volume as compared to the volume of the VOC contaminated gas stream. This makes conveyance of the VOC laden scrubbing liquid significantly easier than conveyance of a VOC contaminated gas stream. In addition, conveyance of the VOC laden scrubbing liquid presents a much smaller risk of leaking and exposure of the VOC to a work environment than conveyance of a VOC contaminated gas stream. Finally, conveyance of a VOC laden scrubbing liquid virtually eliminates a problem of deposits of VOC during conveyance. The method and apparatus further provides for consolidation of VOC laden scrubbing liquid from a number of remotely located sources at a single separating apparatus. This allows for economies of scale in constructing a single large separating apparatus as opposed to many discretely located small separating apparatus. This advantage can be particularly acute when the separation is conducted in a distillation column where efficient operation of the distillation column requires a continuous supply of VOC laden scrubbing liquid. Incorporation of the reservoirs for separated scrubbing liquid and the VOC laden scrubbing liquid provides a buffer insuring a continuous supply of liquid for distillation. Thus, individual liquid absorbers can be brought off-line for servicing or repair without having to shut down the separator. The overall efficiency of the system can be enhanced by providing for heat exchange between the separated scrubbing liquid and the VOC containing scrubbing liquid before the VOC containing scrubbing liquid enters the distillation column. The system is also substantially closed, thereby providing capture of both the VOC and the scrubbing liquid for reuse. Because the method and apparatus are a recovery system, lower VOC removal efficiencies from the VOC contaminated gas streams are permitted by regulation than with disposal systems, thereby making compliance with enviromnental regulations easier.